A wireless communication system has been widely developed to provide various kinds of communication services such as voice and data. Generally, the wireless communication system is a multiple access system that can support communication with multiple users by sharing available system resources (bandwidth, transmission power, etc.). Examples of the multiple access system include a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, and a multi carrier frequency division multiple access (MC-FDMA).
Recently, a radio access network structure has been changed to a structure that various types of small cells, for example, pico cells or femto cells interact with a macro cell. As a cell structure is multi-layered, a data transmission rate and QoE may be improved. In the 3GPP, indoor/outdoor scenarios based on low power nodes to improve small cells have been discussed. This discussion is disclosed in the 3GPP TR 36.932. Also, dual connectivity for a macro cell and the small cells has been discussed. As described above, in the future wireless communication environment, as many small cells are used, it is expected that a user equipment and cells are located at a physically closer distance.